I. Field
This application relates to a two-stage location system, method, and apparatus.
II. Background and Material Information
The location of objects, such as cars, trucks, trains, and even people, can be facilitated using radios equipped with Global Position System (GPS) equipment. For example, a truck may be equipped with a radio that transmits its current position determined using GPS. However, GPS equipment often provides location measurements without sufficient precision for locating an object. For example, if the GPS equipment is in a region with buildings, mountains, or other structures (e.g., such as in a dense urban area), multipath interference may result in imprecise location measurements. In addition, if the GPS equipment is not in view of a sufficient number of GPS satellites (e.g., four GPS satellites), the GPS equipment may provide imprecise location measurements. When GPS equipment is used indoors, it also suffers from precision problems. In situations where precise location measurements are desired, GPS may thus provide poor performance.
Assisted-GPS is an alternative to GPS. Assisted GPS describes a system where an outside source, such as an assistance server, helps a GPS receiver perform the tasks required to determine location. For example, a mobile phone may be equipped with a GPS engine for determining location. The GPS engine in the mobile phone may be “assisted” by an assistance server. The assistance server may use the public wireless network to provide the GPS-enabled mobile phone with information such as an initial location estimate and decoded GPS satellite information (e.g., ephemeris data and clock data). However, the assisted GPS approach is also susceptible to some of the problems mentioned above, such as multipath interference. As such, in situations where precise location measurements are desired, assisted GPS may also provide poor performance.